Fix parsing of (Struct ...) types.

This was a regression that appeared in 5.3.1
(cherry picked from commit 3eb4a75613)

collects/tests/typed-racket/fail/bad-struct-top.rkt

@@ -0,0 +1,7 @@
+#;
+(exn:pred #rx"Argument to Struct must be a structure")
+#lang typed/racket
+
+;; Make sure `Struct` constructor rejects bad arguments
+(: x (Struct Integer))
+

collects/typed-racket/private/parse-type.rkt

@@ -170,9 +170,9 @@
        (add-disappeared-use #'kw)
        (let ([v (parse-type #'t)])
          (match (resolve v)
-           [(and s Struct?) (make-StructTop s)]
+           [(and s (? Struct?)) (make-StructTop s)]
            [_ (tc-error/delayed "Argument to Struct must be a structure type, got ~a" v)
-              (make-Instance (Un))]))]
+              (make-StructTop (Un))]))]
       [((~and kw t:Instance) t)
        (add-disappeared-use #'kw)
        (let ([v (parse-type #'t)])